1. Field of the Invention
The present invention relates to a method of making catalyst, more particularly to a method of making catalyst for cleaning an exhaust gas from an engine.
2. Description of the Prior Art
There has been known a metallic catalyst for cleaning an exhaust gas from an engine wherein catalytic components are impregnated into an alumina coating formed on a metallic carrier. Japanese Patent Publication No. 58-23138 published on May 13, 1983 for opposition, discloses such metallic catalyst. According to the disclosure of the Japanese Patent Publication, as shown in FIG. 6, a corrugated metallic panel 1 is laminated with plane metallic panels 2 at opposite sides to form a honeycomb structure of metallic carrier. Then, on the metallic carrier is formed an alumina coating comprised of .gamma.-alumina and the like. Catalytic components such as platinum, rhodium and the like are impregnated into the alumina coating.
In making such metallic catalyst structure, generally, after forming the aluminum coating on the metallic carrier, the carrier is dipped into an alumina slurry to deposit the alumina on a surface thereof. The carrier is calcined to form an alumina coating 30. Thereafter, the resultant coated carrier is dipped into an aqueous solution including the catalytic components to impregnate the components into the coated carrier. With this method of making catalyst structure, in depositing the alumina coating, a viscosity of the alumina slurry is high enough to deposit through a single step a proper quantity of the alumina into which a desirable quantity of catalytic components can be impregnated. In other words, in this method, a single layer of the alumina coating 30 is formed on the metallic carrier.
It should however be noted that in the case where the alumina coating is formed by a single step of depositing, a thickness of the coating is not uniform across the whole coating. Generally, the thickness of the coating tends to be relatively thick in corner portions A of the carrier structure in FIG. 6 while relatively thin in intermediate portions thereof. This means that even if the desirable quantity of alumina is deposited on the carrier as a whole, the quantity of the alumina may not be sufficient in the intermediate portions of the carrier. It will be understood that a density of the catalytic components is high in the intermediate portions of the carrier relative to the corner portions thereof after impregnation. This partly high density of the catalytic components in the carrier causes a sintering wherein separated elements of the catalytic components are fused to get together in high temperature so that the surface area of the catalyst is reduced resulting in a deterioration of the catalytic activity.